Constrained Ironing

ABSTRACT

Constrained ironing as a new ironing method is proposed for producing thin-walled cans and components with uniform thickness and based on compressive stresses could reach a higher ironing limit ratio or thickness reduction ratio (TRR) without interruption for additional processing such as multi-stage ironing and annealing between different stages. The punch pushes the constrained material to reduce the thickness from the outer surface of the cup. The state of the stresses is fully compressive in this new process while it is tensile in the conventional ironing method.

BACKGROUND OF THE INVENTION

In the conventional ironing process, the drawn cup by a punch through a die with an inner diameter smaller than the outer diameter of the cup is drawn and thin. In practice, to achieve a higher thickness reduction ratio, consecutive dies, each of which has specified diameter is used to determine certain thickness reduction ratio. This method increases the number of steps in ironing that causes problems in process and increases costs. In such multi-stage ironing process is necessary to anneal the pieces between the ironing processes for removing the hardening and improving ductility.

Meanwhile, in multi-dies systems, often punches with long length and high motion of shaft press is required that it makes no coaxial or centrifugal and non-uniformity in the drawn cup. So, in addition to accretion number of steps for forming that needs additional tools and operators, other factors such as pollution and energy consumption increase. This issue was identified as a weakness for the past processes and suggestions to overcome this problem are presented in previous studies. To achieve a higher thickness reduction and to reduce the number of steps in ironing process, extensive research was conducted. The various parameters in the process were analyzed in order to defining optimal parameter values.

Some researchers did some investigations on copper cups in order to achieve the higher thickness reduction through improving the workability of materials by using ultrasonic vibration. Others impose an axial force on the walls of the cup before entering in the deformation zone for improving formability. Some surveys for determining the ratio between extend of ironing and optimum angle of die through derivative equations.

Some used new ironing process entitled “Hydrostatic Ironing” with a very high fluid pressure to achieve a higher thickness reduction ratio which it was 60% of the initial thickness cup. However, their method needs the high fluid pressure about 600 MPa which is supplied by a semi-industrial hydrostatic ironing system. The pressure is very high for industrial applications; moreover, sealing problems and the high cost of machinery equipment are the disadvantages of this method.

Additionally, some surveys have been done where they have proposed “Hydro-ironing” process which provided a higher thickness reduction ratio about 70% through just one step in ironing process. Although this process solve some past problems, the need for annealing the sample after deep drawing to eliminate hardening and improving ductility adds one extra step in the production. It also requires expensive and complex equipment and complex die which could cause problems in the process. Furthermore, supplementation of required pressure could lead to fluid sealing problems. Thus, it is not efficient in mass production and it is not attractive option for artisans.

The Problems in any of the surveys are not fully covered. Therefore, it seems providing a low-cost process, with simple equipment, eliminating annealing step between the ironing process, without limitation in thickness reduction ratio, with minimal tensile stress in deformation zone by applying axial forces and ability to achieve a very high thickness reduction ratio only in one step is necessary for industrial production.

This invention is an appropriate and effective method as an alternative for ironing process which is one of the important processes in manufacturing items that often accompanies with deep drawing. For example, it is used in the production of thin-walled metal cans, cans of spray paint, air filter, a large number of auto parts, CNG capsules, military applications such as producing cartouche and many other applications. All products and parts which are produced by conventional processes can be produced with lower number of manufacturing steps and simpler equipment by this method. This process is an alternative to existing processes and it is more cost-effective.

SUMMARY OF INVENTION

Constrained ironing process has been designed and built for the first time and it does not have any domestic and international sample. The innovative mechanism of this process is the main factor to differentiate it from all available methods.

This process includes:

-   -   1. The use of new geometry and design in pieces of ironing dies         complex (as shown in FIG. 2) with using a piece called “punch”         and a new special geometry of the pieces for imposing force to         the metal cup that looks like none of the usual methods used in         other processes and it is special for the constrained ironing         method (FIG. 2). In this method, deep-drawn metal cup is placed         inside a special die and ironing operations and reducing the         thickness of the deep-drawn cups is done by using the motion of         “punch”. This cylindrical punch can directly receive the         required force for process from press machine.     -   2. Unlike other conventional ironing methods which impose         tensile stress to the cup, in the constrained ironing process,         the nature of stress is compressive. In this manner, the edge of         cylindrical punch imposes force in to the deep-drawn cup and         according to the internal geometry of die, the thickness         reduction about 80 percent occurs in just one step and the         production of metal parts with thin-wall is done. The change in         the nature of the stress to the manufactured item through the         innovative design of the punch makes it possible that we do not         need to ironing ring. According to aforementioned, it can be         added that due to the compressive nature of the process, besides         the ability of ironing ductile parts, ironing of Crisp pieces         and parts which are difficult to deformation are possible as         well. So, various applied type of materials which applied in         industry can be ironed by this process and walls of the items         can be thinned with high thickness reduction ratio.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic of the conventional ironing process and its operation.

FIG. 2 shows a schematic illustration of constrained ironing process at the beginning and during/after the process.

FIG. 3 depicts the map of components of constrained ironing process and their element; where sections a, c) are a punch map and b, d) is a die map.

FIG. 4 shows images of deep-drawn and constrained ironed samples and also shows the thickness distribution in their cross section.

FIG. 5 shows the true engineering stress-strain curve of the annealed and constrained ironed samples (raw and constrained ironed materials).

FIGS. 6A, 6B display the stress contours in the deformation zone can be observed in the constrained ironing process and in traditional ironing process; where A) is constrained ironing process and B) is traditional ironing process.

DETAILED DESCRIPTION

This invention provides a low-cost process, with simple equipment, eliminating annealing step between the ironing process, without limitation in thickness reduction ratio, with minimal tensile stress in deformation zone by applying axial forces and ability to achieve a very high thickness reduction ratio only in one step for industrial production.

The initial idea of the process has originated from the conventional ironing processes and extrusion processes. In the conventional ironing, drawn cup by a punch through a die with an inner diameter smaller than the outer diameter are drawn and thinned materials (FIG. 1).

At first, all drawings were simulated by Abaqus simulation software to reach the maximum amounts of thickness reduction ratio for die and punch through multiple simulations. In the presented method, punch has been designed in a manner that all forces directly impose on it and by applying a compressive force; it has a major role in shaping operations. In this process, at the start, deep-drawn cup is located on the punch, then the punch and metal cup is placed in the die according to the diagram (FIG. 2).

Ironing die includes a knob with a specified angle (Ironing angle). Given that the cylindrical die has a step pattern, the edge of the punch locates on the edge of the deep-drawn cup. So by putting complex under typical hydraulic or electric press and applying force to the punch, metal cup is driven into the ironing die. And according to the internal geometry of die, after passing the cup through the deformation zone, thickness reduction will occur in the cup walls. This process empirically has been applied on deep-drawn cup which are made of aluminum 1050. The design of die is done according to the size which is shown in FIG. 3.

${TRR} = \frac{\left( {t_{1} - t_{f}} \right)}{t_{1}}$

Above formula determines the thickness reduction ratio. t_(i) and t_(f) Define the original and final thickness respectively.

The cross section of deep-drawn an ironed cup are compared in FIG. 4. Metal cup with initial average value in 2.35 mm has been thinned and reach to 0.48 mm thickness through just one step of constrained ironing process; as a matter of fact, the wall thickness of cup is thinned about 80 percent. FIG. 4 shows the distribution of cup's wall thickness which is ironed along the axial direction. Initial wall thickness of top of the cup's wall was 2.5±0.05 and the bottom of the deep-drawn cup was 2.3±0.02 mm, but after the constrained ironing process; they have decreased to 0.48±0.02.

As shown in the FIG. 4, the ironed wall thickness gradually increases with increasing the distance from the base of the cup. In conventional ironing processes, cup's wall are placed under tensile stress while in the constrained ironing process, as cylindrical punch through its step pattern edge imposes compressive force on metal cup, compressive stress is done and severe deformation occurs.

The true stress-strain curve of annealed and ironed of aluminum samples is shown in FIG. 5. The tensile strength into comparison with annealed condition has increased doubly, while increasing the length has decreased sharply as it is expected.

The situation of stress which has been simulated in the deformation zone is visible in FIG. 6. The parameters taken into account for the simulations are shown in Table 1 (Table 1 illustrates applied parameters in constrained ironing process and in conventional ironing process which is used in the simulation process). As can be seen in (FIG. 6.a), the stress situation in the constrained ironing process is fully compressive, while this situation in traditional ironing process is totally stretching.

In the conventional ironing processes, when the shaft axis applied pressure to the punch, punch transmits the pressure to the bottom of the cup and the cup's materials pass from the deformation zone and the wall thickness of the cup is reduced.

Total pressure which is provided by the press consists of two basic parts that some Parts of that involves in friction force and much of it goes to the bottom of the work piece. Applied Pressure on the bottom of the cup causes tensile stress in the wall of the cup and it is the main obstacle in achieving higher thickness reduction ratio, as it causes neck in wall's ironed cup.

Other restrictions of applied tensile stress is that the contraction of cup to the punch and problems in separating them from each other. In traditional methods, increasing friction between the punch and the work piece is a solution to reduce the tensile stress which is provided in the wall of the cup. The constrained ironing method has no stated limitations and due to the nature of fully compressive stress in the deformation zone, it can provide the possibility of intense deformation and high thickness reduction ratio. Compressive stresses lead to Covering the pores and micro-cracks of microstructure of material in cup which can prevent the neck phenomenon.

TABLE 1 Simulation Parameters of constrained and conventional ironing process Values Simulation parameters 2.35 mm (t_(i)) Initial thickness 1.4 mm (t_(f)) Final thickness 10 mm Height of cup's wall (h) 1760 Number of mesh     0.08 Coefficient of (μ) friction 10° (α) Cone angle of die 40% Thickness reduction ratio

The results showed that after constrained ironing process, the tensile strength and hardness increased to 204 MPa and 85 HV, respectively, from the initial values of 71 MPa and 25 HV. Thus, very high TRR is achievable in the constrained ironing process. Obtaining a higher TRR about 80% after only single stage ironing, removing the annealing stages, and obtaining higher strength and hardness of the ironed cup are several advantages of the proposed method. This novel and simple process could be very promising for the industrial applications to replace the conventional process and to reduce the final product cost.

Among the processes that have been introduced, processes that have more thickness reduction ratio conducted ironing operation by hydrostatic pressure and fluid pressure that it increases production equipment, and problems related to sealing of the equipment. In addition, the processes which gain required pressure through fluid are less attractive in industries. This process requires no fluid pressure; takes its power directly from the press machine and hence a very simple and easy process takes place.

Due to the nature of applied compressive stress, crisp metals and metals those are difficult to deform can be ironed by this process. This process is a suitable alternative method for conventional one and the simple method for mass production. Simplicity and ease of process and requires simple equipment. Reduce costs and speed up the production process. For use in many products including beverage cans, there is high chance to compete with other countries in global scale. 

1- A new method of constrained ironing process comprising the steps of: a) Deep-drawn cup is placed on a punch; b) Then said cup and punch are placed inside a special die; c) Applying force to said punch via any type of hydraulic, water or electric press; d) Said cup is forced and driven into said ironing die, wherein during this process walls of said cup are deformed and reduced in thickness. 2- The method of claim 1, wherein said punch is cylindrical and directly receives required force for said constrained ironing process from a press machine. 3- The method of claim 2, wherein an edge of said cylindrical punch imposes force into said deep-drawn cup and according to an internal geometry of said die, said thickness of said deep-drawn cup is at least 80% in just one step of stress to said deep-drawn cup, without a need of an ironing ring. 4- The method of claim 3, wherein said die includes a knob with a specified angle (ironing angle) and said die comprises a step pattern and therefore an edge of said punch fits and sits on an edge of said cup. 5- The method of claim 4, wherein in addition to ironing ductile parts, said process is capable of ironing crisp pieces and parts. 6- The method of claim 5, wherein after constrained ironing process, tensile strength and hardness increased to 204 MPa and 85 HV respectively from an initial value of 71 MPa and 25 HV. 7- The method of claim 6, wherein a TRR (Thickness reduction ratio) is calculated using formula: TRR=(t_(i)−t_(f))/t_(i) wherein t_(i) and t_(f) define an original and final thickness of said cup respectively. 8- The method of claim 7, wherein in said process there is no need for annealing parts between each step of ironing process to improve ductility and removing hardening. 9- The method of claim 8, wherein said final drawn cup comprises a very high strength-to-weight ratio and can be utilized in various industries, comprising but not limited to food packaging industry, automotive and defense industry. 10- The method of claim 9, wherein hardness and strength of said drawn cup increases significantly in comparison to basic raw piece (at least double). 